In recent years, video printers for printing out such an image on a sheet as displayed on a display screen after digitalization are in widespread use. The printing methods of the video printers include thermal method, ink-jet method, laser-beams scanning method, liquid crystal shutter method, and so forth. Among others, the liquid crystal shutter method is closely watched in view of its suitability for downsizing and weight saving.
The liquid crystal shutter method is a method for forming the image on a photosensitive member continuously and relatively moving, by controlling light irradiation to the photosensitive member with the use of a liquid crystal shutter device which controls light transmittance for each of a plurality of pixels aligned in one or more line(s) by ON/OFF, that is, opening/closing, by applying or not applying voltages to a liquid crystal layer. A printer employing the method is referred to as an optical printer.
A printing method using such a liquid crystal shutter device for printing out the image on a photosensitive paper using full range of colors is disclosed for example in JP, S62-134624, A. This method is characterized in that the liquid crystal shutter device and a white light source are used, in which a turret with color filters of red, blue, and green thereon is rotated by a motor to thereby selectively irradiate light from the light source as lights of red, green, or blue to the photosensitive paper via the liquid crystal shutter device, so that a full-color image is formed on the photosensitive paper.
Also, in JP, H6-186581, A, a structure to be hereinafter described is presented as one structure of the liquid crystal shutter device, in which pixel rows corresponding to the respective three light colors are provided, where pixels composing the pixel rows are aligned in two lines for each color in a direction orthogonal to a moving direction of the photosensitive paper at same pitches as of a pixel size of the aligning direction, each pixel of one of the two lines and that in the other line being arranged misaligning with each other by one pixel size in respective aligning directions, and a lead-out electrode of each pixel being led out in the moving direction of the photosensitive paper through spaces between the pixels.
When configuring the optical printer using the liquid crystal shutter device described in the latter patent gazette, there is required no adjustment in timings to rotate color filters, to open/close the pixels in the liquid crystal shutter device, and to move the photosensitive paper, enabling to form an extremely precise image. It is also possible to form the image by the pixels of the other line even in the spaces between the pixels which are required for wirings, enabling to form a high-quality and streak-free image.
However, when adopting the structure of the liquid crystal shutter device described in the latter patent gazette, there has been a problem that the lead-out electrodes for the center pixel row, namely the pixel row arranged as a second color, are wired by detouring around perimeters of pixel electrodes of first or third color pixels arranged at outer sides, so that driving waveform for the second color pixels affects open/close of the first or third color pixels to thereby cause an orthogonal band-shaped image irregular with respect to the relative moving direction of the photosensitive paper.
Hence, the configuration and the problem of the conventional liquid crystal shutter device will be described using FIG. 28 to FIG. 30 and FIG. 7. FIG. 28 is a plan view showing a conventional liquid crystal shutter device, FIG. 29 is a plan view for illustrating wirings of electrodes in the liquid crystal shutter device shown in FIG. 28, FIG. 30 is a sectional view taken along line 30—30 in FIG. 29, and FIG. 7 is a graph for depicting characteristics of the liquid crystal shutter device. Note that the pixels shown in these drawings are quite larger than their actual sizes for convenience of illustration.
As shown in FIG. 28 to FIG. 30, this liquid crystal shutter device has a transparent first substrate 1 and a transparent second substrate 21 adhered with a given gap therebetween by a sealant 33, and the gap is filled with liquid crystal and sealed with the sealant 33 and a closing member 34 to thereby hold the liquid crystal between the substrates 1 and 21 as a liquid crystal layer 32. In the optical printer employing this liquid crystal shutter device, as light colors to be irradiated to a photoreceptor, three colors of red (R), green (G), and blue (B) are used, and pixel rows are arranged in the liquid crystal shutter device to correspond to respective colors, as shown in FIG. 28. A single pixel row at the center is denoted by G pixel row 300, and two rows of an R pixel row 200 and a B pixel row 400 are arranged at the outer sides thereof, the respective pixel rows being composed of a plurality of pixels aligned in two lines as mentioned before.
Focusing on the respective pixels, as shown in FIG. 29 and FIG. 30, as signal electrodes made from a transparent conductive film, there are arranged, on the first substrate 1, the pixel electrodes for forming: the first line pixels of the R pixel row 200, such as an R1a pixel 2, an R1b pixel 4 which is arranged apart from the R1a pixel 2 by one pixel size in the aligning direction, and the like; and the second line pixels of the R pixel row 200 such as an R2a pixel 3 and the like which are arranged apart a little from the first line pixels by misaligning with the first line pixels by one pixel size at the same pitches. In FIG. 29, a reference number 11 representatively denotes an R1a pixel electrode forming the R1a pixel 2, and only part of the pixels are shown in the drawing for convenience of illustrating.
Additionally, the R1a pixel electrode 11 for example connects with an R1a connecting electrode 16 via an R1a lead-out electrode 15 to thereby connect with a first driving integrated circuit (IC) 61 via the R1a connecting electrode 16, whereby it is possible to apply driving signals to the R1a pixel electrode 11 from the first driving IC 61. The same is equally true of the other pixel electrodes and whereby it is possible to apply driving signals from the first driving IC 61 to each pixel electrode. Besides, the first driving IC 61 is connected with a first FPC 63 via a not-shown anisotropic conductive film provided on the first substrate 1, and the first FPC 63 applies required signals from outside to the first driving IC 61.
Further, there are arranged the pixel electrodes for forming the first line pixels of the G pixel row 300 at a position apart from the second line of the R pixel row 200 farther than the space between the lines of the R pixel row 200. A G1a pixel electrode 13 for example for forming a G1a pixel 7 connects with a G1a connecting electrode 17 via a G1a lead-out electrode 18 to thereby connect with the first driving IC 61 via the G1a connecting electrode 17. The same is equally true of the other pixel electrodes, and their lead-out electrodes detour around the respective pixel electrodes which form the pixels of the R pixel row 200 and are led out through the spaces therebetween to the respective connecting electrodes so that the other pixel electrodes connect with the first driving IC 61 via the lead-out electrodes and the connecting electrodes.
Furthermore, there are arranged the pixel electrodes for forming the second line pixels of the G pixel row 300 such as a G2a pixel 8 and the like. Moreover, at a position apart therefrom in the same distance as of the R pixel row 200 second line and the G pixel row 300 first line, there are arranged the pixel electrodes for forming the first line pixels of the B pixel row 400 such as a B1a pixel 9 and the like, and the second line pixels of the B pixel row 400 such as a B2a pixel 10 and the like.
These pixel electrodes are arranged substantially symmetric with respect to the pixel electrodes for forming the first and second line pixels of the R pixel row 200 and the first line pixels of the G pixel row 300, and connect with a second driving IC 62 provided on the counter side of the first driving IC 61 via the lead-out electrodes led out orthogonally to the aligning direction of the pixel electrodes and the connecting electrodes. Also, here, the lead-out electrodes led out from the pixel electrodes for forming the second line pixels of the G pixel row 300 provided in the center side detour around the pixel electrodes for forming the pixels of the B pixel row 400 and pass through the spaces therebetween to thereby connect with the respective connecting electrodes. Similarly, the second driving IC 62 connects with a second FPC 64 via a not-shown anisotropic conductive film provided on the first substrate 1, so that the second FPC 64 applies required signals from outside to the second driving IC 62.
Meanwhile, as a common electrode made of a transparent conductive film, there is provided a counter electrode 28 on the liquid crystal layer 32 side surface of the second substrate 21. Inside the sealant 33, the counter electrode 28 is provided in a rectangular shape so as to face all pixel electrodes for forming the pixels of respective pixel rows to thereby connect with an RGB pad electrode 47 provided outside of the sealant 33.
With such pixel electrodes and counter electrode 28, this liquid crystal shutter device achieves a high contrast ratio, in which a static drive being one liquid crystal driving method for increasing the response speed of the liquid crystal layer 32 is performed.
Also, on the counter electrode 28, there is provided a black matrix (BM) 24 as a light shield film. The black matrix 24 is provided inside the sealant 33 and slightly inside the counter electrode 28 so as to directly border on the counter electrode 28. Accordingly, when using a conductive light shield film such as a metal film and the like for the black matrix 24, the counter electrode 28 and the black matrix 24 have the same electric potential. In the black matrix 24, there are still provided BM openings 29 in portions corresponding to the respective pixel electrodes provided on the first substrate 1, the BM opening 29 being smaller than the pixel electrode in area. The overlapped portion of the pixel electrode and the BM opening 29 forms the pixel where the amount of transmitted light is practically controlled.
Still, on the opposite side surface of the liquid crystal layer 32 of the first substrate 1, a first polarizer 71 is provided, and on the opposite side surface of the liquid crystal layer 32 of the second substrate 21, a second polarizer 73 is provided. The voltages applied to the liquid crystal layer 32 by the respective pixel electrodes and the counter electrode 28 are changed, and the transmitting state of light rays 75 transmitting through the pixel portions is controlled by the first polarizer 71, the second polarizer 73, and the liquid crystal layer 32, so that irradiated light amount to the not-shown photosensitive member is controlled.
In the conventional liquid crystal shutter device mentioned before, the counter electrode 28 is formed substantially all over the inside of the sealant 33 on the second substrate 21 without regard to the arrangement of pixel rows. Hence, not only the pixel electrodes but also the lead-out electrodes inevitably face the counter electrode, and therefore when applying voltages to between the respective pixel electrodes and the counter electrode 28, the voltages with the same electrical potentials are also applied to between the lead-out electrodes corresponding to the pixel electrodes and the counter electrode 28. As a result, driving signals to one pixel affect the voltages to be applied to the liquid crystal layer 32 at the other pixel via the counter electrode 28, and thus affect the transmittance of the other pixels in the end.
As with the example described above, when the three pixel rows are provided corresponding to the three colors of lights, the lead-out electrodes corresponding to the pixels of the inner side pixel row (here, G pixel row 300) are led out longer, so that their areas facing the counter electrode 28 become larger, besides, they are led out through near the pixel electrodes and the lead-out electrodes corresponding to the pixel rows of the outer sides (here, the R pixel row 200 and the B pixel row 400), so that the driving signals to the pixels of the inner side pixel row strongly affect the transmittance of the pixels of the outer side pixel rows.
In this regard, more detailed description will be provided. In the conventional liquid crystal shutter device mentioned above, a structure can be adopted in which, at 0th (level of) tone, a time period during which voltages having a larger absolute value is applied to the liquid crystal layer 32 is at the minimum and the transmittance is at the maximum, at 255th tone, the time period during which voltage having a larger absolute value is applied to the liquid crystal layer 32 is at the maximum and the transmittance is at the minimum, and 127th tone is seen as a medium. In such a structure, after fixing the signals to be applied for example to the R1a pixel 2 to 127 tones signals, if tones of the first line pixels of the G pixel row, of which lead-out electrodes are led out around the R pixel row side, are shifted from 127 tones states to 255 tones sequentially pixel by pixel, the transmittance gradually changes despite the voltages to be applied to the R1a pixel 2 are fixed. A curving line X in FIG. 7 shows this change.
In FIG. 7, the horizontal axis represents the number of first line pixels of the G pixel row which have been shifted from 127th tone to 255th tone, and the vertical axis represents the change in the transmittance at the focused R1a pixel 2. As shown in the graph, if such one G pixel as connected with the lead-out electrode provided between the R1a pixel 2 and the R1b pixel 4 in the first line is shifted from 127th tone to 255th tone, the transmittance in the focused R1a pixel 2 goes away from the target values. Similarly, if an adjacent G pixel of the same line is shifted from 127th tone to 255th tone, the transmittance at the focused R1a pixel 2 goes further away from the target values. When the number of the G pixels shifted from 127th tone to 255th tone is increased to 10, the transmittance at the focused R1a pixel 2 changes by about 3% from the initial transmittance at 127th tone. This means that the exposed amount at one pixel deviates from a target exposure amount by the same percentages, when the photoreceptor is exposed.
When the pixels of the G pixel row connected with the lead-out electrodes passing through the vicinity of the other pixels of the R pixel row are shifted in tone, the transmittance of the pixels of the R pixel row similarly deviates from the target values. These deviations appear as a band-shaped image irregular on a printed paper. Such a problem occurs also between the second color pixels (G pixels) and the third color pixels (B pixels), resulting in the similar band-shaped image irregular.
This band-shaped image irregular is specifically distinct when the pixels of respective colors are away from each other, and the image irregular still remains when they are close to each other, even though it is indistinctive. Besides, when the pixels of respective colors are closely provided, it is still difficult to separate colors, and optical system becomes complicated. Specifically, when increasing light amount, a light source becomes larger, and thus it is difficult to provide them closely.
In its essence, in the conventional liquid crystal shutter device as shown in FIG. 28 to FIG. 30, there is such a problem that the pixels of the two outer side rows are largely affected on their light transmittance when signals are applied to the center row pixels.
Should the liquid crystal shutter device of a matrix type be employed here, the above-described influence on the image irregular can be lessened, while response speed and contrast ratio also fall to thereby cause printing time increase and image quality down. In addition, it is required to closely provide the light sources of the respective colors, so that light interference and color mixture occur. Even though the light sources are turned on in sequence for preventing color mixture, printing time and the light amount volatility of the light sources increase as compared to the case where the light sources are turned on simultaneously for printing, since the lights are turned on for example in the order of red, green, and blue in a time-shared manner.
It is an object of the invention to bring solutions to such a problem and make the liquid crystal shutter device provided with a plurality of pixel rows capable of controlling light transmittance of the pixels composing the pixel rows to a desired value and controlling light irradiation to the photosensitive member appropriately, so that a high quality of image without image irregular can be formed.